This invention relates to the evaluation of radiation therapy phantoms in particular to a phantom using film and providing radiation measurement throughout a volume.
External beam radiation therapy treats cancerous tissue by exposing the tissue to a high-energy radiation from an external source. Normally, a number of different external beams are employed, each approaching the tissue at a different angle, simultaneously or in sequence. The use of multiple beams and angles minimizes the radiation exposure of any given area of the skin and of nearby, possibly radiation-sensitive organs. The selection of the angles and the exposure times for each beam comprises a radiation treatment plan.
Whereas some treatment plans may have a relatively low number of beams and exposure times, the latest generation of radiation therapy equipment allows for extremely complex radiation treatment plans employing many independently controllable beams throughout a range of angles. Multiple beams of varying average intensity may be formed by a multileaf collimator or similar mechanism.
Such complex radiation treatment plans provide precise placement of dose upon tumor tissue, but place severe demands on phantoms used to verify the dose produced by the treatment plan. A conventional radiation therapy phantom incorporates an attenuating material, such as plastic or water, interacting with radiation in a manner equivalent to that of human tissue. One or more radiation detectors, for example, ionization detectors or flat sheets of radiation sensitive film are located within the attenuating material to measure the radiation at different locations.
Conventional phantoms are cumbersome or expensive when accurate characterization of a dose throughout a volume is required, requiring repeated measurements and repositioning of the phantom or its detectors. Accordingly, the present inventors have developed a “spiral” phantom using a single sheet of radiation sensitive film rolled in a spiral to provide dose measurements in a volume rather than a single plane. Knowledge of the mathematical description of the spiral and the properties of the material in which the spiral is cut, allows the radiation measured by the film at different locations upon its two dimensional surface to be related to the doses at different volumes within the three dimensions of the phantom. The spiral phantom is particularly useful for complex intensely modulated radiation therapy protocols and is described in the article: “Spiral Phantom for IMRT and Tomotherapy Treatment Delivery Verification” by Bhudatt Paliwal and Wolfgang Tomé, Susan Richardson and T. Rockwell, Med. Phys. 27(11), November 2000, pp. 2503-2507. These papers are hereby incorporated by reference.
As noted in this paper, although the prototype spiral phantom provided good qualitative assessment of the treatment plan, deviation in the prediction of dose and in the measured dose of the spiral phantom, particularly at the outer arm of the spiral, limited its use in precise quantitative applications.